When people think of yolks, they often think of the nutrient rich parts of eggs which are used to provide nutrition to creatures like birds and reptiles as they develop. However, mammals also develop yolks, in the form of a yolk sac, one of the earliest structures to appear in the course of embryonic development. Pregnant women are often interested in the sac because it is usually the first thing to become visible on ultrasound, while doctors like to use it to gauge embryonic development.
Much like the yolk of a bird's egg, the yolk sac is designed to provide nutrition to the developing embryo. This structure first becomes visible in ultrasounds at about five weeks. As the embryo grows, much of the sac is eventually absorbed into its body, and it gets nutrients from its mother. The vestiges of the sac can be seen in the afterbirth. Women who are interested can ask to see their afterbirth and have the various components explained.
In the very early stages of pregnancy, the body forms what is known as the gestational sac, which envelops the embryo as it develops. When an ultrasound is performed, the sac is usually the most noticeable feature, with the yolk sac being positioned between the embryo and the uterine wall. A doctor may perform measurements of the yolk sac to gather information about the pregnancy and its progress. Irregularities in the size of the sac can indicate that a problem is occurring, and the mother may be at increased risk of miscarriage or complications.
As the embryo is developing and taking nourishment from the yolk sac, which also acts as a crude circulatory system, the placenta between the embryo and the uterine wall is also developing. Eventually, the placenta will be able to take over the duties of supplying nutrients and handling blood circulation. At this point, the contents of the sac are more or less depleted.
When an ultrasound is performed in the early weeks of pregnancy to confirm that a pregnancy is occurring and that conditions appear to be normal, the technician will usually be happy to point out the various structures which can be seen on the screen or in the image. He or she can show the expecting mother where the sac is, and provide an estimate of how far along she is on the basis of the size of the sac and other factors.